To further characterize the function of myostatin, in vivo metabolic assays were performed including euglycemic-hyperinsulinemic clamp studies, lipid oxidation and indirect calorimetry using wild type and myostatin null mice. We have also measured the effects of high fat and normal diets on insulin and glucose tolerance and the effects of exercise on myostatin mull mice as compared to wild type mice. To determine whether GDF11 and myostatin have redundant functions in skeletal muscle size control, we have analyzed muscle mass in mice carrying a deletion of GDF11 specifically in skeletal muscle in a myostatin null background.